The present invention relates in general to metering valves for dosing liquids or gases and in particular to a fuel injection valve for use in fuel injection systems of internal combustion engines, for example in direct injection for diesel engines and the like. The metering valve is of the type which includes a valve housing provided with a metering opening, a valve needle passing through the metering opening and being connected at its free end with a valving element, a biasing spring arranged between the housing and the opposite end of the needle to urge the valving element into its closing position against a valve seat around the metering opening, piezoelectric control member arranged in the housing in alignment with the needle, another biasing spring for urging the piezoelectric control member against the opposite end of the needle, and damping means arranged in the housing and cooperating with the piezoelectric control member to compensate for position changes of the member relative to the housing.
In a known measuring valve of this kind (GB Publication No. 2,056,559) the arrangement of the piezoelectric control member, the needle and the valving element is designed such that the valving element is displaced from the valve seat in response to the contraction of a column of stacked piezoelectric elements. Biasing spring supported on the housing and arranged in a damping space communicating with a fuel inlet engages the side of the damping piston which is remote from the piezoelectric stack and urges the valve needle with its valving element into its closing position. The mass of the damping piston and the damping effect of fuel in the damping space are adjusted such that during the short contraction phase of the piezoelectric stack the damping piston due to the inertia of its mass remains stationary and does not move in the valve closing direction even if subject to the valve closing spring. Consequently, in each dosing phase the valve needle and its valving element always perform the same opening stroke. If the valve is closed the piezoelectric stack due to temperature fluctuations exhibits corresponding changes in its length and the damping piston is shifted accordingly. In the same manner, wear and manufacturing tolerances of the operational system formed by the valve needle, piezoelectric stack and the damping piston are compensated for and cannot affect the regulating distance of the valve needle relative to the metering opening of the valve.
However, it has been found that the prior art design for fixing the momentary position of the piezoelectric stack in the housing during the dosing phase cannot be applied to those embodiments where the extension of the piezoelectric stack is used for actuation of the valve needle and its valving element. In such cases pressure acting on the damping piston due to extension of the piezoelectric stack causes a perceptible shift of the damping piston in spite of the relatively minute total displacement of the piezoelectric stack amounting typically to 30 microns. This displacement of the piston is caused by the fact that fuel present in the damping space moves practically without damping and consequently it flows very fast through the connection conduit to the fuel inlet. Only at a strong throttling of the discharged fuel a momentary displacement of the damping piston due to the expanding piezoelectric stack can be prevented provided that the damping piston has a very large mass. Such massive damping pistons however require a relatively large installation space which would interfere with the desired small volume of the metering valve. On the other hand, even a very small shift of the fixed position of the piezoelectric stack relative to the housing during the measuring phase due to the small controlling movements of the piezoelectric stack prevents a highly accurate and reliably reproducible fuel dosing.